1. Field of the Invention
The present invention relates to a detector for detecting the level of an output of a transmitter, such as a mobile radio transmitter, for transmitting, as its output, a high-frequency signal, and a transmitter incorporating the detector.
2. Description of the Prior Art
Referring now to FIG. 4, it illustrates a schematic circuit diagram showing the structure of a prior art detector used for detecting the level of an output of a transmitter for transmitting, as its output, a high-frequency signal. In the figure, reference numeral 41 denotes an input terminal to which the high-frequency signal to be detected by the detector is applied, numeral 42 denotes a capacitor having an end connected to the input terminal 41, and numeral 43 denotes a detecting diode having an anode connected to another end of the capacitor 42. In addition, reference numeral 44 denotes a first resistor having an end connected to a junction between the capacitor 42 and the detecting diode 43, and another end connected to a bias supplying unit 45 for supplying a predetermined bias voltage to the detecting diode, numeral 48 denotes a capacitor having an end connected to a cathode of the detecting diode 43 and a detected voltage output terminal 46, and another end connected to a ground, and numeral 49 denotes a second resistor having an end connected to the cathode of the detecting diode 43 and the detected voltage output terminal 46, and another end connected to the ground. The first and second resistors 44 and 49 define a bias current flowing through the detecting diode 43. The capacitor 48 is charged by the high-frequency signal, which has been half-wave rectified by the detecting diode 43.
The prior art detector as shown in FIG. 4 is so constructed as to make a bias current flow through the detecting diode 43 in order to detect a high-frequency signal at a relatively low level. The high-frequency signal to be detected is supplied from the input terminal 41, by way of the capacitor 42, to the anode of the detecting diode 43. The capacitor 42 serves as a bypass capacitor to cut off the DC component of the high-frequency signal applied to the detector and to allow its high-frequency components to pass therethrough.
Unless any high-frequency signal is applied to the input terminal 41, the bias current supplied from the bias supplying unit 45 flows through the first resistor 44, the detecting diode 43, and the second resistor 49. The bias current has a value determined by the quotient of (the bias voltage from the bias supplying unit 45- the forward voltage VF of the detecting diode 43) by the sum of the resistances of the first and second resistors 44 and 49.
When a high-frequency signal is applied to the input terminal 41, the high-frequency signal passes through the bypass capacitor 42 and the sum of the voltage Vin of the high-frequency signal and the forward voltage VF is then applied to the detecting diode 43. The waveforms of instantaneous currents flowing through the detecting diode 43 in the positive and negative halves of each cycle of the input high-frequency signal are asymmetrical to each other because the conductance of the detecting diode 43 largely varies according to the direction of the voltage across the detecting diode.
The capacitor 48 is charged by the instantaneous current flowing through the detecting diode in the positive half of each cycle. In the next negative half of each cycle, the capacitor 48 discharges and the instantaneous current therefore flows through the second resistor 49 to the ground. As a result, the total current flowing through the resistance component of the detected voltage output terminal 46 is the sum of the bias current supplied from the bias supplying unit 45 and the instantaneous current caused by the high-frequency signal. Thus the voltage signal that appears at the detected voltage output terminal 46 has a value corresponding to the level of the input high-frequency signal.
A problem with the prior art detector shown is that it has a detection characteristic showing temperature dependence, that is, the detecting diode 43 has a conductance that can vary with temperature. A change in the forward voltage VF with temperature can cause a change in the detected voltage that appears at the detected voltage output terminal 46 even though the same bias current is caused to flow through the detector.
Referring next to FIG. 5, it illustrates a schematic circuit diagram showing the structure of another detector as disclosed in Japanese Patent Application Publication (TOKKAIHEI) No. 8-330850, which is proposed to solve the above problem. In the figure, the same reference numerals as shown in FIG. 4 denote the same components as of the former prior art detector, and therefore the description of these components will be omitted hereinafter. In FIG. 5, reference numeral 47 denotes a temperature-compensation diode having an anode connected to the cathode of a detecting diode 43, and a cathode connected to an end of a resistor 49, numeral 50 denotes a resistor having an end connected to a junction between the detecting diode 43 and the temperature-compensation diode 47, and another end connected to a detected voltage output terminal 46, numeral 51 denotes a resistor having an end connected to the cathode of the temperature-compensation diode 47, and another end connected to the detected voltage output terminal 46, and numeral 52 denotes a capacitor having an end connected to a junction between the detecting diode 43 and the temperature-compensation diode 47, and another end connected to a ground. The capacitor 52 serves as a bypass capacitor for preventing a high-frequency voltage supplied to the detecting diode 43 from being supplied to the temperature-compensation diode 47. Two diodes having the same characteristics can be used as the detecting diode 43 and the temperature-compensation diode 47. For example, Schottky barrier diodes encapsulated in the same package are used. The first and second resistors 44 and 49 have the same resistance value.
Assuming that the sum of the resistance values of the third and fourth resistors 50 and 51 is sufficiently greater than the resistance value of the temperature-compensation diode 47 at its operating point, the bias current, which flows through the series circuit consisting of the first resistor 44, the detecting diode 43, the temperature-compensation diode 47, and the second resistor 49, has a value determined by the quotient of {the bias voltage from the bias supplying unit 45- (the forward voltage VF of the detecting diode 43+ the forward voltage VF of the temperature-compensation diode 47)} by the sum of the resistance values of the first and second resistors 44 and 49 when a high-frequency signal is applied to the input terminal 41. Since the forward voltage VF of each of the detecting and temperature-compensation diodes 43 and 47 has a negative temperature coefficient, the bias current has a positive temperature coefficient. Similarly, the voltage that appears at a junction c between the temperature-compensation diode 47 and the resistor 49 has a positive temperature coefficient. On the other hand, the voltage that appears at a junction b between the detecting diode 43 and the temperature-compensation diode 47 has a constant value equal to one-half of the bias voltage because the junction b sits right at the center of the series circuit consisting of the first and second resistors 44 and 49, and the two diodes 43 and 47. As a result, the DC offset voltage at the detected voltage output terminal 46 has a positive temperature coefficient because it has a value corresponding to the division of the voltage between the junctions b and c using the third and fourth resistors 50 and 51.
When a high-frequency signal is applied to the input terminal 41, the high-frequency signal passes through the bypass capacitor 42 and the sum of the voltage Vin of the high-frequency signal and the forward voltage VF is then applied to the detecting diode 43, as previously mentioned. The waveforms of instantaneous currents flowing through the detecting diode 43 in the positive and negative halves of each cycle of the input high-frequency signal are asymmetrical to each other because the conductance of the detecting diode 43 largely varies according to the direction of the voltage across the detecting diode. As a result, the average current increases and a DC component therefore emerges. The high-frequency components of the instantaneous current pass through the bypass capacitor 52 and then flows into the ground. On the other hand, the bypass current increases the voltage that appears at the junction b between the detecting diode 43 and the temperature-compensation diode 47, and an envelope signal integrated by the resistor 50 and the capacitor 48 emerges at the detected voltage output terminal 46.
The conductance of each of the detecting and temperature-compensation diodes 43 and 47 has a negative temperature coefficient, and the detected voltage that appears at the detected voltage output terminal 46 therefore has a negative temperature coefficient. Since the total voltage that appears at the detected voltage output terminal 46 is the sum of the DC offset voltage having a positive temperature coefficient and the detected voltage having a negative temperature coefficient, it is possible to make the detection characteristics of the prior art detector remain unchanged with temperature by setting the ratio of dividing the voltage between the junctions b and c using the resistors 50 and 51 to a proper value.
However, a problem with the prior art detector constructed as above is that the magnitude of the detected output is reduced. Since in the prior art detector as shown in FIG. 5 the detected output obtained from the detected voltage output terminal 46 is the division of a voltage detected by the detecting diode 43 using the third and fourth resistors 50 and 51, the detected output inherently has a lower value than the detected voltage. In the prior art detector, a larger input power has to be applied to the input terminal 41 to increase the voltage of the detected output, and therefore there is a need to increase the degree of coupling of a directional coupler disposed at the front of the detector. However, an increase in the degree of coupling can increase losses produced between a power amplifier and a transmission antenna included with a transmitter incorporating the detector, thus reducing the magnitude of an output transmitted via the transmission antenna and hence increasing the power consumption. Accordingly, a detector capable of furnishing an output having as high a voltage as possible for a limited input power, that is, a high-efficiency detector is best suited to transmitters. The prior art detector as shown in FIG. 5 cannot meet such a demand.